Having high energy density, fuel cells with polymer electrolyte membranes are expected to serve as power supplies or convenient auxiliary power supplies for mobile devices, household cogeneration systems, and automobiles, using methanol, hydrogen or the like as fuel. Development of polymer electrolyte membranes with excellent properties is one of the most critical aspects of the fuel cell technology.
In a fuel cell with a polymer electrolyte membrane, the electrolyte membrane works to conduct protons and serves as a diaphragm that prevents direct mixing of the fuel hydrogen or methanol with the oxidant air (oxygen). The membrane needs to have the following properties to work as an electrolyte membrane: high ion-exchange capacity; excellent chemical stability, as electric current is to be applied for a long period of time, especially high resistance (oxidation resistance) to hydroxide radicals and the like, which are main factors that cause the membrane to deteriorate; heat resistance at cell operating temperature, that is, 80° C. and above; and constant and high water retention in order to keep electrical resistance low. To serve as a diaphragm, on the other hand, the electrolyte membrane is required to have excellent mechanical strength and dimensional stability and not to allow excessive hydrogen gas, methanol, or oxygen gas to pass through the membrane.
Early fuel cells using polymer electrolyte membranes employed a hydrocarbon-containing polymer electrolyte membrane produced by copolymerization of styrene and divinylbenzene. However, this electrolyte membrane was not so practical because it was very poor in durability due to its oxidation resistance; later, use of perfluorosulfonic acid-containing membranes such as Nafion® developed by DuPont has become popular.
Although the conventional fluorine-containing polymer electrolyte membranes such as Nafion® have excellent chemical durability and stability, they are low in ion-exchange capacity, approximately 1 mmol/g, and insufficient in water retention. Thus, there have been problems that ion-exchange membranes dry out to cause a decrease in proton conductivity and that, in the case in which methanol is used as fuel, swelling of the electrolyte membranes and/or cross-over of the methanol occur.
There has been another problem. Since no crosslinked structure is introduced in the polymer chains, introduction of. a greater amount of sulfonic acid groups in order to increase the ion-exchange capacity leads to a significant decrease in strength due to swelling of the membrane. This causes the membrane to break easily. Therefore, with the conventional fluorine-containing polymer electrolyte membranes, the amount of sulfonic acid groups needs to be adjusted such that the strength of the membrane is maintained. Thus, it has only been possible to produce membranes with an ion-exchange capacity of approximately 1 mmol/g.
In the field of graft polymerization using ionizing radiation, which is a technique related closely to the present invention, it has been tried to produce solid polymer electrolyte membranes by a process in which monomers to which sulfonic acid groups are introducible are graft-polymerized on polymer membranes.
The present inventors have studied to develop the new solid polymer electrolyte membranes. As a result, they have developed a solid polymer electrolyte membrane that is characterized by controllability of ion-exchange capacity over a wide range and is producible by first performing radiation-induced grafting polymerization to introduce styrene monomers into a polytetrafluoroethylene film to which a radiation-induced crosslinked structure is. only introducible at a temperature of 340±5° C. in an inert gas atmosphere, and then performing sulfonation, as well as a process for producing this membrane (Patent Document 1). The polymer electrolyte membrane, however, has a problem. Since the styrene graft chains in the polymer electrolyte membrane are composed of hydrocarbons, part of the graft chains becomes oxidized when electric current is applied to the membrane for a long period of time. This lowers the ion-exchange capacity of the membrane.
The present inventors have studied in view of this problem. As a result, they have developed a solid polymer electrolyte membrane that is characterized by a large ion-exchange capacity and excellent oxidation resistance and is developed by radiation-induced graft polymerization or radiation-induced graft copolymerization of fluorine-containing monomers on a polytetrafluoroethylene film having a crosslinked structure and then introducing sulfonic groups into the resulting graft chains, as well as and a process for producing the membrane (Patent Document 2). However, it has been found that, with an ordinary fluorine-containing polymer membrane used as the polymer substrate, the graft polymerization of the fluorine-containing monomers does not proceed to an inner part of the membrane easily and, depending on the reaction conditions, the graft polymerization is effective only on a surface of the substrate. Therefore, it is difficult to provide electrolyte membranes having improved in properties.
The present inventors have studied further to advance the processing technologies using radiation. As a result, they have developed a process for producing an electrolyte membrane that is characterized by having better oxidation resistance performance than the conventional membranes, which process is characterized in that ethylene-tetrafluoroethylene copolymers to which a radiation-induced crosslinked structure is easily introducible in an inert gas atmosphere at a temperature close to room temperature or other polymers as a partially-fluorinated polymer film substrate are used (polymer film substrates containing a main chain in which a hydrocarbon bonded to a fluorocarbon appears as a repeating unit, e.g., polyvinylidene fluoride having —CH2—CF2— as a repeating unit), and styrene derivatives or multicomponent monomers containing styrene derivatives are introduced into an inner part of the substrate by use of radiation-induced grafting, and then molecular chains that the substrate has, graft molecular chains, and the graft chains and the molecular chains that the substrate has are re-irradiated with radiation to introduce a multi-crosslinked structure, and finally sulfonation is performed (Japanese Patent Application No. 2005-170798).
Having the multi-crosslinked structure between the main chain and the graft molecular chains that the polymer film substrate has, the thus prepared polymer electrolyte membrane is significantly improved in oxidation resistance. However, when the polymer electrolyte membrane undergoes cell operation for a long period of time at high temperature, part of the graft molecular chains deteriorates due to swelling or suffers a decrease in water retention. This leaves a problem in terms of use at practical level.    [Patent Document 1] JP 2001-348439 A    [Patent Document 2] JP 2002-348389 A